LINQ Contains

Summary: in this tutorial, you’ll learn how to use the LINQ Contains() method to check if a sequence contains a specific element.

Introduction to the LINQ Contains() method

The Contains() is a LINQ extension method that returns true if a sequence contains an element:

bool Contains<TSource>(
    this IEnumerable<TSource> source, 
    TSource value
);Code language: C# (cs)

In this syntax:

TSource is the type of element in the source sequence.
source is the input sequence with the IEnumerable<T> type.
value is the element to locate in the source sequence.

The Contains() method returns true if the source sequence contains the element value or false otherwise.

Once the Contains() find the value in the source sequence, it stops enumerating the sequence immediately.

The Contains() uses a default equality comparer to compare the value with the elements in the source sequence.

The Contains() method is available for all collections that implement the IEnumerable<T> interface including List<T>.

If you want to use a custom equality comparer, you can use the following method overload:

bool Contains<TSource>(
    this IEnumerable<TSource> source, 
    TSource value,
    IEqualityComparer<TSource>? comparer
);Code language: C# (cs)

In this syntax, you can pass a custom equality comparer to the third parameter of the Contains() method to compare the value with the elements of the source sequence.

LINQ Contains() examples

Let’s take some examples of using the LINQ Contains() method.

1) Using the LINQ Contains() to check if an array contains a value

The following program demonstrates how to use the LINQ Contains() method to check if an array contains a specific integer:

using static System.Console;

int[] numbers = { 1, 2, 3, 4, 5 };

bool containsThree = numbers.Contains(3);
bool containsTen = numbers.Contains(10);

WriteLine(containsThree); // true
WriteLine(containsTen); // falseCode language: C# (cs)

In this example, we use the Contains() method to check if the numbers array contains the integers 3 and 10 respectively.

The method returns true for 3 because it is present in the array and false for 10 because it is not available in the array.

2) Using the LINQ Contains() to check if a list contains a value

The following program illustrates how to use the LINQ Contains() method to check if a list of strings contains a string:

using static System.Console;

List<string> fruits = new(){ 
    "apple", 
    "banana", 
    "cherry",
    "strawberries"
};

bool containsCherry = fruits.Contains("cherry"); 
bool containsMango = fruits.Contains("mango"); 

WriteLine(containsCherry);// true
WriteLine(containsMango);// falseCode language: C# (cs)

In this example, we use the Contains() method to check if the fruits list contains the string cherry and mango respectively.

The method returns true for cherry because cherry is present in the list while it returns false for mango because mango is not in the list.

3) Using the LINQ Contains() method that use a custom equality comparer

The following example demonstrates how to use the LINQ Contains() method to check if a person is in the list of Person objects using a custom equality comparer:

// Define a custom Person class
class Person
{
    public string? SSN { get; set; }
    public string? Name { get; set; }
}

// Define a custom PersonComparer class that compares
// Person objects based on their SSN
class PersonComparer : IEqualityComparer<Person>
{
    public bool Equals(Person? x, Person? y)
    {
        // Check for null values
        if (x == null || y == null)
            return false;

        // Check if the two Person objects are the same reference
        if (ReferenceEquals(x, y))
            return true;

        // Compare the SSN of the two Person objects
        // to determine if they're the same
        return x.SSN == y.SSN;
    }

    public int GetHashCode(Person? obj)
    {
        if (obj == null || obj.SSN == null)
            return 0;

        // Use the SSN of the Person object
        // as the hash code
        return obj.SSN.GetHashCode();
    }
}
class Program
{
    static void Main(string[] args)
    {
        // Create a List of Person objects
        List<Person> people = new List<Person> {
            new Person { SSN = "123-45-6789", Name = "John Smith" },
            new Person { SSN = "987-65-4321", Name = "Jane Doe" },
            new Person { SSN = "555-55-5555", Name = "Bob Johnson" }
        };

        // Create a Person object to check if it's present in the List
        Person person = new() { SSN = "123-45-6789", Name = "John Smith" };

        // Check if the List contains the Person object using a custom comparer
        bool containsPerson = people.Contains(person, new PersonComparer());

        if (containsPerson)
        {
            Console.WriteLine($"{person.Name} is present in the list of people.");
        }
        else
        {
            Console.WriteLine($"{person.Name} is not present in the list of people.");
        }

        Console.ReadLine();
    }
}
Code language: C# (cs)

Output:

John Smith is present in the list of people.Code language: C# (cs)

How it works.

First, define a Person class that has two properties: Social Security Number (SSN) and Name:

class Person
{
    public string? SSN { get; set; }
    public string? Name { get; set; }
}Code language: C# (cs)

Second, define a custom PersonComparer class that compares two Person objects based on their SSN:

class PersonComparer : IEqualityComparer<Person>
{
    public bool Equals(Person? x, Person? y)
    {
        // Check for null values
        if (x == null || y == null)
            return false;

        // Check if the two Person objects are the same reference
        if (ReferenceEquals(x, y))
            return true;

        // Compare the SSN of the two Person objects
        // to determine if they're the same
        return x.SSN == y.SSN;
    }

    public int GetHashCode(Person? obj)
    {
        if (obj == null || obj.SSN == null)
            return 0;

        // Use the SSN of the Person object
        // as the hash code
        return obj.SSN.GetHashCode();
    }
}Code language: C# (cs)

Third, create a list that contains three Person objects and a person object to locate in the list. The Contains() method checks if the list contains the person object using the PersonComparer that compares two person objects by their SSN. The method returns true because the person object with the name John Smith and SSN "123-45-5555" is in the list:

class Program
{
    static void Main(string[] args)
    {
        // Create a List of Person objects
        List<Person> people = new List<Person> {
            new Person { SSN = "123-45-6789", Name = "John Smith" },
            new Person { SSN = "987-65-4321", Name = "Jane Doe" },
            new Person { SSN = "555-55-5555", Name = "Bob Johnson" }
        };

        // Create a Person object to check if it's present in the List
        Person person = new() { SSN = "123-45-6789", Name = "John Smith" };

        // Check if the List contains the Person object using a custom comparer
        bool containsPerson = people.Contains(person, new PersonComparer());

        if (containsPerson)
        {
            Console.WriteLine($"{person.Name} is present in the list of people.");
        }
        else
        {
            Console.WriteLine($"{person.Name} is not present in the list of people.");
        }

        Console.ReadLine();
    }
}
Code language: C# (cs)

Summary

Use the Contains() method to check if a sequence contains an element using an equality comparer.

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